How to dynamically launch and kill nodes?

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Is there any best practice and/or guides surrounding how to dynamically launch and kill nodes?

I'm building a small Raspberry Pi based robot, and since it only has 1GB of memory and limited CPU, I don't want every possibly node I use to be loaded, because even a node that's "standing by" consumes about 2-3% of memory.

For example, say I want to start a procedure that looks for QR codes in a video stream. If I don't normally do this, it doesn't make sense to have a QR tracking node running at all times, so I'll want to dynamically launch the node as needed, and then kill it when the procedure is complete. What's the best way to do this using rospy?